Radiators come in all sorts of shapes and sizes.  They will be assembled in any number of lengths, depending on what the heat load of the room requires.  There are instances where it is not physically possible to get a radiator out of a building in one piece because the radiator is just too big.  When it was originally installed, it would have been hoisted in during construction or it arrived in pieces and was assembled on site.

When are radiator is too large or too heavy to be safely removed, it can be disassembled and taken out in multiple pieces if required. Again, from a delivery and installation perspective, if a very large rad needs to be installed back into a building, it can be done in several pieces and reassembled and pressure tested on site to ensure it is ready for installation.